mysql中单表查询的相关方法

单表查询

前期表数据准备

# 下面的代码测试都是基于这个表的

create table emp(
  id int not null unique auto_increment,
  name varchar(20) not null,
  sex enum('male','female') not null default 'male', #大部分是男的
  age int(3) unsigned not null default 28,
  hire_date date not null,
  post varchar(50),
  post_comment varchar(100),
  salary double(15,2),
  office int, #一个部门一个屋子
  depart_id int
);

#插入记录
#三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('jason','male',18,'20170301','张江第一帅形象代言',7300.33,401,1), #以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('nick','male',18,'19000301','teacher',30000,401,1),
('sean','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

表中数据展示：

1、语法执行顺序

# 简单查询语句
select id,name from emp where id>=3 and id<=6;
# 执行顺序
from  # 读取文件
where  # 筛选条件
select  # 显示指定结果

# 补充：查看数据时另一种数据展现方式(解决字段太多，终端横向距离不够显示的问题)
# select * from emp \G;
mysql> select * from emp \G;
*************************** 1. row ***************************
          id: 1
        name: jason
         sex: male
         age: 18
   hire_date: 2017-03-01
        post: 张江第一帅形象代言
post_comment: NULL
      salary: 7300.33
      office: 401
   depart_id: 1
*************************** 2. row ***************************
          id: 2
        name: egon
         sex: male
         age: 78
   hire_date: 2015-03-02
        post: teacher
post_comment: NULL
      salary: 1000000.31
      office: 401
   depart_id: 1
*************************** 3. row ***************************
          id: 3
        name: kevin
         sex: male
         age: 81
   hire_date: 2013-03-05
        post: teacher
post_comment: NULL
      salary: 8300.00
      office: 401
   depart_id: 1
.........剩下的数据.........

2、where 约束条件

1.查询id大于等于3小于等于6的数据
# 方式一：
select * from emp where id>=3 and id<=6;

# 方式二：
select * from emp where id between 3 and 6;


2、查询薪资是20000或者18000或者17000的数据
# 方式一
select * from emp where salary=20000 or salary=18000 or salary=17000;

# 方式二
select * from emp where salary in (20000,18000,17000);  # 简写


3.查询员工姓名中包含o字母的员工姓名和薪资
# 思路：
    在刚开始接触mysql查询的时候，建议先按照查询的优先级顺序拼写出你的sql语句
'''
1、先查那张表 from emp
2、根据什么条件去查 where name like '%o%'
3、再对得到的结果进行筛选显示  select name,salary
'''
=> 
# select name,salary from emp where name like '%o%';
mysql> select name,salary from emp where name like '%o%';
+-------+------------+
| name  | salary     |
+-------+------------+
| jason |    7300.33 |
| egon  | 1000000.31 |
| owen  |    2100.00 |
+-------+------------+
3 rows in set (0.00 sec)


4.查询员工姓名是由四个字符组成的员工姓名与其薪资
# 方式一
select name,salary from emp where id like '____';

# 方式二
select name,salary from emp where char_length(id)=4;


5、查询薪资不在20000，18000，17000范围的数据
select * from emp where salary not in (20000,18000,17000);


6、查询岗位描述为空的员工名与岗位名  针对null不能用等号，只能用is
# select name,post from emp where post_comment = null;
mysql> select name,post from emp where post_comment = null;
Empty set (0.00 sec)

# select name,post from emp where post_comment is null;
mysql> select name,post from emp where post_comment is null;
+-----------+-----------------------------+
| name      | post                        |
+-----------+-----------------------------+
| jason     | 张江第一帅形象代言             |
| egon      | teacher                     |
| kevin     | teacher                     |
| tank      | teacher                     |
| owen      | teacher                     |
| jerry     | teacher                     |
| nick      | teacher                     |
| sean      | teacher                     |
| 歪歪      | sale                        |
| 丫丫      | sale                        |
| 丁丁      | sale                        |
| 星星      | sale                        |
| 格格      | sale                        |
| 张野      | operation                   |
| 程咬金    | operation                   |
| 程咬银    | operation                   |
| 程咬铜    | operation                   |
| 程咬铁    | operation                   |
+-----------+-----------------------------+
18 rows in set (0.00 sec)

# 也可以查post_comment不为空的数据
# select name,post from emp where post_comment is not null;
mysql> select name,post from emp where post_comment is not null;
Empty set (0.00 sec)

3、补充知识：exist(了解即可)

exist关键字用来标识 是否存在
在使用exist关键字是，内层查询语句不返回查询的记录，而是返回一个布尔值，若内层能取到数据则返回True，否则返回False，与之对应的是外层执不执行查询操作
# 代码示例：
select * from emp where exist (select * from emp where 1=2);
select * from emp where exist (select * from emp where id>1);

4、group by

数据分组应用场景：每个部门的平均薪资，男女比例

1、按部门分组
select * from emp group by post;
# 非分组的严格模式下会取出每个分组的第一条数据
# 严格模式下会直接报错，因为一旦分组了，就无法再‘直接’查到其他子弹的信息了，只能获取到分组的组名

# 设置分组为严格模式：
set global sql_mode="strict_trans_tables,only_full_group_by";
# 设置完需要重新连接客户端

测试：
select * from emp group by post;  # 报错
select id,name,sex from emp group by post;  # 报错
select post from emp group by post;  # 成功
# 成功代码示例：
mysql> select post from emp group by post;
+-----------------------------+
| post                        |
+-----------------------------+
| operation                   |
| sale                        |
| teacher                     |
| 张江第一帅形象代言          |
+-----------------------------+
4 rows in set (0.00 sec)

强调：只要分组了，就不能够再“直接”查找到单个数据信息了，只能获取到组名


# 通过聚合函数可以查找到其他数据
2、获取每个部门的最高工资
# 以组为单位统计组内数据>>>聚合查询(聚合到一起合为一个结果)
select post,max(salary) from emp group by post;
# 代码示例：
mysql> select post,max(salary) from emp group by post;
+-----------------------------+-------------+
| post                        | max(salary) |
+-----------------------------+-------------+
| operation                   |    20000.00 |
| sale                        |     4000.33 |
| teacher                     |  1000000.31 |
| 张江第一帅形象代言          |     7300.33 |
+-----------------------------+-------------+
4 rows in set (0.01 sec)

# 同理可得：
# 每个部门最低工资
select post min(salary) from emp group by post;

# 每个部门平均工资
select post avg(salary) from emp group by post;

# 每个部门的工资总和
select post sum(salary) from emp group by post;

# 每个部门的人数
select post count(id) from emp grop by post;


3、查询分组后的部门名称 和 每个部门下所有的学生姓名
# group_concat（分组了之后才会用）不仅可以用来显示除分组外字段还有拼接字符串的作用
3.1）select post,group_concat(name) from emp group by post;  # 显示除分组外的字段
# 代码示例：
mysql> select post,group_concat(name) from emp group by post;
+-----------------------------+------------------------------------------------+
| post                        | group_concat(name)                             |
+-----------------------------+------------------------------------------------+
| operation                   | 张野,程咬金,程咬银,程咬铜,程咬铁                    |
| sale                        | 歪歪,丫丫,丁丁,星星,格格                          |
| teacher                     | egon,kevin,tank,owen,jerry,nick,sean           |
| 张江第一帅形象代言             | jason                                          |
+-----------------------------+------------------------------------------------+
4 rows in set (0.00 sec)

3.2）select post,group_concat(name,"_SB") from emp group by post;  # 显示除分组外的字段并且拼接字符串
# 代码示例：
mysql> select post,group_concat(name,"_SB") from emp group by post;
+-----------------------------+-------------------------------------------------------------+
| post                        | group_concat(name,"_SB")                                    |
+-----------------------------+-------------------------------------------------------------+
| operation                   | 张野_SB,程咬金_SB,程咬银_SB,程咬铜_SB,程咬铁_SB                 |
| sale                        | 歪歪_SB,丫丫_SB,丁丁_SB,星星_SB,格格_SB                        |
| teacher                     | egon_SB,kevin_SB,tank_SB,owen_SB,jerry_SB,nick_SB,sean_SB   |
| 张江第一帅形象代言             | jason_SB                                                    |
+-----------------------------+-------------------------------------------------------------+
4 rows in set (0.00 sec)


3.3）select post,group_concat(name,": ",salary) from emp group by post;
# 代码示例：
mysql> select post,group_concat(name,": ",salary) from emp group by post \G;
# 字段与记录以字段的冒号分隔对齐
*************************** 1. row ***************************
                          post: operation
group_concat(name,": ",salary): 张野: 10000.13,程咬金: 20000.00,程咬银: 19000.00,程咬铜: 18000.00,程咬铁: 17000.00
*************************** 2. row ***************************
                          post: sale
group_concat(name,": ",salary): 歪歪: 3000.13,丫丫: 2000.35,丁丁: 1000.37,星星: 3000.29,格格: 4000.33
*************************** 3. row ***************************
                          post: teacher
group_concat(name,": ",salary): egon: 1000000.31,kevin: 8300.00,tank: 3500.00,owen: 2100.00,jerry: 9000.00,nick: 30000.00,sean: 10000.00
*************************** 4. row ***************************
                          post: 张江第一帅形象代言
group_concat(name,": ",salary): jason: 7300.33


3.4）select post,group_concat(salary) from emp group by post;
# 代码示例
mysql> select post,group_concat(salary) from emp group by post;
+-----------------------------+-------------------------------------------------------------+
| post                        | group_concat(salary)                                        |
+-----------------------------+-------------------------------------------------------------+
| operation                   | 10000.13,20000.00,19000.00,18000.00,17000.00                |
| sale                        | 3000.13,2000.35,1000.37,3000.29,4000.33                     |
| teacher                     | 1000000.31,8300.00,3500.00,2100.00,9000.00,30000.00,10000.00|
| 张江第一帅形象代言             | 7300.33                                                     |
+-----------------------------+-------------------------------------------------------------+
4 rows in set (0.00 sec)


4.补充concat（不分组时用）拼接字符串达到更好的显示效果 as语法使用
# as语法基本使用：
select name as '姓名',salary as '薪资' from emp;
# 代码示例：
mysql> select name as '姓名',salary as '薪资' from emp;
+-----------+------------+
| 姓名      | 薪资       |
+-----------+------------+
| jason     |    7300.33 |
| egon      | 1000000.31 |
| kevin     |    8300.00 |
| tank      |    3500.00 |
| owen      |    2100.00 |
| jerry     |    9000.00 |
| nick      |   30000.00 |
| sean      |   10000.00 |
| 歪歪      |    3000.13 |
| 丫丫      |    2000.35 |
| 丁丁      |    1000.37 |
| 星星      |    3000.29 |
| 格格      |    4000.33 |
| 张野      |   10000.13 |
| 程咬金    |   20000.00 |
| 程咬银    |   19000.00 |
| 程咬铜    |   18000.00 |
| 程咬铁    |   17000.00 |
+-----------+------------+
18 rows in set (0.00 sec)


# as 与 concat 合用
select concat('NAME:',name) as '姓名',concat('SALARY:',salary) as '薪资' from emp;
# 代码示例：
mysql> select concat('NAME:',name) as '姓名',concat('SALARY:',salary) as '薪资' from emp;
+----------------+-------------------+
| 姓名           | 薪资              |
+----------------+-------------------+
| NAME:jason     | SALARY:7300.33    |
| NAME:egon      | SALARY:1000000.31 |
| NAME:kevin     | SALARY:8300.00    |
| NAME:tank      | SALARY:3500.00    |
| NAME:owen      | SALARY:2100.00    |
| NAME:jerry     | SALARY:9000.00    |
| NAME:nick      | SALARY:30000.00   |
| NAME:sean      | SALARY:10000.00   |
| NAME:歪歪      | SALARY:3000.13    |
| NAME:丫丫      | SALARY:2000.35    |
| NAME:丁丁      | SALARY:1000.37    |
| NAME:星星      | SALARY:3000.29    |
| NAME:格格      | SALARY:4000.33    |
| NAME:张野      | SALARY:10000.13   |
| NAME:程咬金    | SALARY:20000.00   |
| NAME:程咬银    | SALARY:19000.00   |
| NAME:程咬铜    | SALARY:18000.00   |
| NAME:程咬铁    | SALARY:17000.00   |
+----------------+-------------------+
18 rows in set (0.00 sec)


# 补充as语法 即可以给字段起别名也可以给表起
select emp.id,emp.name from emp as t1; # 报错  因为表名已经被你改成了t1
select t1.id,t1.name from emp as t1;  # 成功


# 查询四则运算
# 查询每个人的年薪
select name,salary*12 as annual_salary from emp;
select name,salary*12 annual_salary from emp;  # as可以省略

5、having

# 需求：先分类，在筛选
截止目前已经学习的语法：
select 查询字段1,查询字段2,... from 表名
    where 过滤条件
    group by分组依据

# 语法这么写，但是执行顺序却不一样
from
where  # 筛选
group by  # 分组
select

# 可得：where的筛选进行的 顺序在分组之间，而我们想在分好组后的数据中进行筛选==>having
having的语法格式与where一致，只不过having是在分组之后进行的过滤，即where虽然不能用聚合函数，但是having可以！

1、统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门
select post,avg(salary) from emp
        where age >= 30
        group by post
        having avg(salary) > 10000;
# 代码示例：
mysql> select post,avg(salary) from emp
    ->     where age >= 30
    ->     group by post
    ->     having avg(salary) > 10000;
+---------+---------------+
| post    | avg(salary)   |
+---------+---------------+
| teacher | 255450.077500 |
+---------+---------------+
1 row in set (0.00 sec)

# 如果不信你可以将having取掉，查看结果，对比即可验证having用法！
# 代码示例:
mysql> select post,avg(salary) from emp
    ->     where age >= 30
    ->     group by post;
+---------+---------------+
| post    | avg(salary)   |
+---------+---------------+
| sale    |   2500.240000 |
| teacher | 255450.077500 |
+---------+---------------+
2 rows in set (0.00 sec)


#强调：having必须在group by后面使用
select * from emp having avg(salary) > 10000;  # 报错

6.distinct

# 对重复的数据进行去重操作
select distinct post from emp;
# 代码示例：
mysql> select distinct post from emp;
+-----------------------------+
| post                        |
+-----------------------------+
| 张江第一帅形象代言             |
| teacher                     |
| sale                        |
| operation                   |
+-----------------------------+
4 rows in set (0.00 sec)

7.order by

# 将数据按薪资从大到小排序
select * from emp order by salary asc;  # 默认升序
select * from emp order by salary desc;  # 降序排

select * from emp order by age desc; #降序排

#先按照age降序排，在age相同的情况下再按照薪资升序排
select * from emp order by age desc,salary asc; 

# 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，然后对平均工资进行排序
select post,avg(salary) from emp
    where age > 10
    group by post
    having avg(salary) > 1000
    order by avg(salary);
    
# 代码示例：
mysql> select post,avg(salary) from emp
    ->     where age > 10
    ->     group by post
    ->     having avg(salary) > 1000
    ->     order by avg(salary);
+-----------------------------+---------------+
| post                        | avg(salary)   |
+-----------------------------+---------------+
| sale                        |   2600.294000 |
| 张江第一帅形象代言             |   7300.330000 |
| operation                   |  16800.026000 |
| teacher                     | 151842.901429 |
+-----------------------------+---------------+
4 rows in set (0.00 sec)

8.limit

# 限制展示条数
select * from emp limit 3;
# 查询工资最高的人的详细信息
select * from emp order by salary desc limit 1;

# 分页显示
select * from emp limit 0,5;  # 第一个参数表示起始位置，第二个参数表示的是条数，不是索引位置
select * from emp limit 5,5;  # 第一个参数：从第5条记录开始显示，第二个参数：往后显示5条

9.正则

% _
正则(regexp)
贪婪匹配：  .*    
与非贪婪匹配:  .*?

# 匹配 以 j 开头，以 n或y 结尾 的记录
select * from emp where name regexp '^j.*(n|y)$';
# 代码示例：
mysql> select * from emp where name regexp '^j.*(n|y)$' \G;
*************************** 1. row ***************************
          id: 1
        name: jason
         sex: male
         age: 18
   hire_date: 2017-03-01
        post: 张江第一帅形象代言
post_comment: NULL
      salary: 7300.33
      office: 401
   depart_id: 1
*************************** 2. row ***************************
          id: 6
        name: jerry
         sex: female
         age: 18
   hire_date: 2011-02-11
        post: teacher
post_comment: NULL
      salary: 9000.00
      office: 401
   depart_id: 1
2 rows in set (0.00 sec)

转载于:https://www.cnblogs.com/shuchengyi/articles/10870893.html